Solenoid-operated pilot type spool valves that switch a spool with a solenoid operated pilot valve are, as disclosed in Patent Literature 1, for example, already known. Such kinds of spool valve include a double pilot operated spool valve having two pilot valves, and a single pilot operated spool valve having a single pilot valve.
Among the above, in a double pilot operated spool valve, pilot air is alternately made to act on pilot pistons that are provided on both sides of a spool by alternately turning the two pilot valves on and off to reciprocate the spool so that flow passages between a plurality of ports are switched.
On the other hand, in a single pilot operated spool valve, pilot pistons having diameters that are different from each other are disposed on one end and the other end of a spool, and by supplying and discharging pilot air to and from the pilot piston with a large diameter by turning the pilot valve on and off while the pilot air is made to act on the pilot piston with a small diameter, the flow passages between a plurality of ports are switched by reciprocation of the spool using the difference in acting force according to the difference in pressure receiving areas between the two pilot pistons.
As disclosed in Patent Literature 1, typically, in the spool valve of the above kind, piston boxes are connected to both ends of a valve body built in with a spool, the pilot pistons are accommodated inside the piston boxes, and pilot air is guided from an input port formed in the valve body to the piston boxes through a pilot passage that passes through the inside of the valve body such that the pilot air is made to act on the pilot pistons through the passages formed inside the piston boxes.
Accordingly, in the above-described conventional spool valves, the pilot passage needs to be further formed inside the valve body which is provided with a plurality of ports for input, output, and exhaustion, depressions and holes for attaching various parts, and a plurality of fitting holes for fitting to a manifold base, and as a result, the internal structure of the valve body becomes complex leading to a problem of complexity in design and fabrication.